The Food and Agricultural Organization of the United Nations define probiotics as “live microorganisms which when administered in adequate amounts confer a health benefit on the host”. Nowadays, a number of different bacteria are used as probiotics for example, lactic-acid producing bacteria such as strains of Lactobacillus and Bifidobacteria. 
Lactic-acid producing bacteria are not only used for their beneficial effect on human or animal health, but they are also widely used in the food industry for fermentation processes. The effectiveness of probiotics is strain-specific, and each strain may contribute to host health through different mechanisms. Probiotics can prevent or inhibit the proliferation of pathogens, suppress production of virulence factors by pathogens, or modulate the immune response in a pro-inflammatory or an anti-inflammatory way. Use of different strains of the probiotic lactic-acid producing bacteria Lactobacillus reuteri is a promising therapy for the amelioration of infantile colic, alleviation of eczema, reduction of episodes of workplace illness, and suppression of Helicobacter pylori infection. L. reuteri is considered an indigenous organism of the human gastrointestinal tract and is present for example on the mucosa of the gastric corpus, gastric antrum, duodenum, and ileum. See, for example, U.S. Pat. Nos. 5,439,678, 5,458,875, 5,534,253, 5,837,238, and 5,849,289.
When L. reuteri cells are grown under anaerobic conditions in the presence of glycerol, they produce the antimicrobial substance known as reuterin (β-hydroxy propionaldehyde).
The relationship between a host and its microbes is complex, and for some bacteria, this host: microbe relationship has been developing over many years of co-evolution. This appears to be especially true for Lactobacillus reuteri. Our knowledge of the mutualistic relationship between gut microbes and the human host is in its infancy, but already we are keenly aware that the gut microbiome plays an essential role in gut and immune system development, nutrition, and new links are being established between the gut microbiome and the brain. Dysbiosis, the perturbation of the normal gut microbiome, has been implicated in a wide range of disease processes including those affecting the local gut environment, such as Inflammatory Bowel Disease (IBD) and Irritable Bowel Syndrome (IBS), and disease processes at sites distant to the gut, such as the metabolic syndrome. Significant therapeutic potential lies within the gut microbiome, and research is striving towards a future goal of altering the microbial community in order to prevent and/or treat distinct disease processes.
There is therefore a need to understand such specific interactions between microbes and man related to a specific disease or other situations influencing the health of the host so that the most appropriate probiotic strains can be selected and used to counteract such developments.